As it is well known, exhaust gases from such engines contain many pollutants such as unburnt hydrocarbons (HC), carbon monoxide, nitrogen oxides (NO and NO2), more commonly referred to as NOx, as well as particles.
It is widely agreed that NOx emissions result from the combustion that occurs at high temperatures and in the presence of oxygen. These conditions are generally encountered in any type of combustion, in particular those taking place under lean burn conditions, such as direct injection in lean burn mode, whatever the fuel used.
Now, NOx emissions involve a major drawback as they have a harmful effect directly on human health, in particular NO2, and indirectly through the secondary formation of tropospheric ozone.
In order to comply with the emissions standards and to preserve the environment and human health, it has become necessary to treat these pollutants prior to discharging the exhaust gas to the atmosphere.
As it is generally well known, this is achieved by means of a treatment for depolluting the exhaust gas circulating in the exhaust line of the engine.
Thus, in order to treat the unburnt hydrocarbons and the carbon monoxide from engines running with a lean mixture, catalysis means such as an oxidation catalyst are arranged on the exhaust line.
As regards exhaust gas, in particular for a Diesel engine, a particle filter is advantageously arranged on this line so as to capture and to eliminate the particles present in the exhaust gas, and thus to avoid discharging them to the atmosphere.
This filter, which can also be a catalysed filter, needs to be periodically regenerated in order to keep all of its filtration capacities by achieving combustion of the particles retained in this filter. These regeneration operations mainly consist in increasing the filter temperature, which may either occur spontaneously when using the engine at high load, or be generated by an exothermic oxidation, on a catalyst arranged upstream from the filter, of reducing chemical species resulting from the combustion or from an injection directly into the exhaust, triggered by the engine control.
As regards the NOx emissions, the exhaust gas also flows through other catalysis means, notably catalysts of SCR (Selective Catalytic Reduction) type. This SCR catalyst allows to selectively reduce the NOx to nitrogen through the action of a reductant.
This reductant, which is generally injected upstream from the SCR catalyst, can be ammonia or a compound generating ammonia by decomposition, such as urea, or a hydrocarbon from a hydrocarbon-containing substance, oxygenated or not.
Currently, the commonest technique for NOx depollution is SCR catalysis using ammonia.
This ammonia is indirectly obtained by decomposition of a precursor injected in liquid form, generally a 32.5 mass % aqueous urea solution, better known under the brand name “AdBlue” or “DEF”.
Thus, the urea solution is injected into the exhaust line upstream from the SCR catalyst. The water contained in this solution is rapidly vaporized under the effect of the exhaust gas temperature, then each urea molecule decomposes in two stages into two ammonia molecules:(NH2)2CO (urea)→NH3 (ammonia)+HNCO (isocyanic acid)  (1)HNCO+H2O→NH3+CO2  (2)
Alternatively, ammonia can be directly injected in gas state into the exhaust line upstream from the SCR catalyst.
As described in more detail in document EP-2,541,012, the exhaust gas depollution system comprises an exhaust line including a depollution means that combines a particle filter and a selective catalytic reduction (SCR) catalyst, this assembly being referred to as SCR catalysed filter or SCRF filter, a single tank containing a mixture of an additive for particle filter regeneration and of a reductant for elimination of the NOx present in this depollution means, and an injector for feeding this mixture upstream from the SCRF filter.
Although this system is satisfactory, it however involves not insignificant drawbacks.
Indeed, the system described in document EP-2,541,012 is limited to the use of a regeneration additive containing an oxygen storage capacity material.
Such an additive offers few advantages in an already oxygen-rich medium, such as an exhaust line of a lean running engine.
Besides, it is described that the catalytic phase of the SCR catalyst is protected only when it is coated in a particle filter.
This excludes in fact configurations where the particle filtration and NOx catalytic reduction by SCR functions are performed on dissociated elements.
Furthermore, the described system only concerns situations where the exhaust gas temperature is high.
Therefore, combustion of the particles is likely to further increase the temperature within the SCRF filter, which may lead to catalytic phase deterioration.
The present invention aims to overcome the aforementioned drawbacks by means of a product and a method allowing exhaust gas depollution to be carried out in a simple and effective manner.